A DC/DC converter is a circuit that receives a continuous DC voltage at the input and efficiently converts it into a continuous DC voltage of a generally different value, at the output. The input voltage can be constant or variable. A DC/DC converter is used when a linear regulator does not offer reasonable efficiency for the application. For instance, if a fresh battery has an initial voltage Uo, that decreases to 0.6Uo, as the battery discharges, the maximum regulated voltage that a linear regulator can provide, is below 0.6Uo. This fact by itself is enough to make a linear regulator unacceptable in numerous applications. Furthermore, when the battery is fresh, its voltage is Uo and the output voltage is below 0.6Uo. This means that almost one-half of the power delivered by the battery is dissipated in the linear regulator, resulting in less than 60% efficiency, which renders a linear regulator unacceptable in numerous applications.
There are many prior art DC/DC converter topologies which, unlike a linear regulator, all involve switching of the DC input voltage. Examples of such converter topologies include step-up or boost converters, whose output voltage is greater than the input voltage, step-down or buck converters, whose output voltage is smaller than the input voltage, boost-buck converters, etc. Many attempts have been made to improve the efficiency, response time, range of operation, etc., for these circuits. Essentially, these circuits receive a voltage input and efficiently convert it to a regulated voltage output.
Unlike DC/DC converters, AC/DC converters receive an alternative voltage at the input and efficiently convert it into a continuous voltage, at the output. There are many possible architectures for AC/DC converters, depending on the application. Generally, a transformer receives an AC primary voltage signal and produces an AC secondary voltage signal, which is then rectified, filtered and regulated to provide a desired output voltage. If the frequency of the input signal does not allow good overall efficiency and/or poses problems from performance, practical device values and/or cost points of view, the input signal can first be rectified and filtered and then followed by a DC/DC converter to produce the desired regulated output. These prior art architectures may be combined in various manners, such as to optimize performance, cost, size, ease of implementation, etc
Applications arise in which the performance parameter of interest at the output of the DC/DC converter or AC/DC converter (hereinafter referred to as a DC converter) depends more on the output current than on the output voltage. If the DC converter circuit uses one or more inductors, it is referred to herein as an inductive DC converter. DC converters are essentially voltage sources. In order to control the output current, said output current must be sensed and the corresponding signal fed back to the DC converter, which will adjust the output voltage for a desired value of output current. An example of a load whose performance depends essentially on the load current, instead of the load voltage, is a light emitting diode (LED). Imaging device coils, solenoid actuators, bulbs, etc., are further examples of loads that are current driven. The luminous output of an LED depends, essentially, on the current flowing in the LED. When temperature varies, at constant current, the forward voltage drop across the diode varies. Because of that, a constant voltage applied to the diode would lead to a luminous output which follows the current in the diode and varies with temperature. Instead, a constant current feeding the LED generates a constant luminous output.
As long as the load impedance is linear, a DC converter which uses the load current as its feedback signal can, in principle, effectively regulate the load current. However, if the impedance is either very large or very small, it might become difficult to (a) sense the current and/or (b) to provide reasonable feedback signal levels to the DC converter. If a small voltage variation across the load induces a large current variation in the load, it becomes difficult to regulate the load current using prior art DC converters. Non-linear load impedances present an even more important and more difficult task of load current regulation. All semiconductor diodes, not only LEDs, present a strongly non-linear characteristic of exponential dependency of the current, on the voltage across the diode. That is, a small variation in the voltage across the diode generates a large variation in the current flowing through it. Many other loads, including discrete components and more complex loads, exhibit non-linear characteristics and/or impedance.
However, there is a negative consequence of driving a load directly with a DC converter having a voltage regulated output, when it is desired to regulate the load current. Any DC regulator will exhibit overshoot or undershoot at power up and when the load changes. This is an intrinsic feature of the control loop, which can be minimized to a certain degree, but not eliminated. The voltage overshoot or undershoot at the output of the DC converter is reflected in the load current, thus degrading load current regulation.
Another negative consequence is observed when driving non-linear loads directly with a DC converter. In the case of a diode load, the initial current in the device is practically zero, until the applied voltage reaches the forward voltage drop value for that particular diode. Until this happens, the DC converter senses there is no current in the load, while it tries to regulate that current to a certain non-zero value. So the DC converter ramps up the output voltage hard, but the current is still zero. The process continues, with the DC regulator core reaching maximum output power, until the voltage is high enough to turn on the diode. Actually, at all times, the diode impedance is exponential, but, for all practical purposes, because the corresponding current levels are so small, it appears there is no current flowing into the diode, until the forward voltage reaches a certain value, after which the voltage will remain almost constant for the entire operating current range of the diode. In reality, the voltage across the diode increases a small amount, as the current increases, but the exponential characteristic translates a very large current variation into a very small voltage variation. Hence, the whole practical range of currents in the diode correspond to a very small range of voltages across the diode; that is, the voltage is nearly constant when the diode is on. In this example, when the diode turns on, it is very likely that a large overshoot in load current will occur, because the DC converter feedback loop is operating at maximum output power and it needs time to resume regulation, once there is sensible current flow in the load. Frequency compensation, such as to overdamping the loop response, leads to slower loop response and, consequently, to poorer regulation performance for the DC converter